Detection of a biological or chemical target in a sample using a detectable label is a procedure at the heart of many biological detection methods, including medical diagnostic methods. In some cases the target may be a particular polynucleotide sequence or gene, a mutation of a gene, a genetic expression pattern, detected at the DNA or RNA level, either in situ or after extraction or isolation. In other cases, the target may be a peptide, protein, antigen, or other substance, again detected in situ or after isolation or laboratory manipulation. The target may also be a particle or debris of organic origin.
Many standard detection methods, e.g. IHC, ISH, ELISA, blotting, etc., employ labeling schemes to detect the desired targets. Typically, those schemes involve incubating an experimental sample potentially containing the detectable target with a probe, and then detecting the binding between binding agent and target with a detectable label which may give off a color, a fluorescent signal, or radioactivity, for example. One or many binding agent molecules may bind to each target, depending upon the specifics of the scheme used. In some cases, especially when the target is present in low concentration, it is necessary to amplify the signal from the target-binding agent complex by adding one or more amplification layers to the system. For example, if the binding agent is a primary antibody that recognizes the target, a secondary antibody that recognizes the primary antibody may be added such that many secondary antibodies bind to each primary antibody. If the secondary antibodies are attached to a detectable label such as a fluorophore or chromophore, then, via amplification, each target molecule in the sample may effectively be bound to multiple fluorophores or chromophores instead of only one or a few fluorophores or chromophores. Hence, the target will produce a stronger detection signal after amplification.
Some detection experiments, however, have a tendency to produce relatively diffuse-looking signals, especially if the sample is allowed to rest for a period of time before analysis. For example, the one or more binding agents and/or detectable labels bound to a target may slowly diffuse away from the target, or away from each other over time. In some cases buffer changes that affect the binding affinity of the target, binding agent, and amplification layers can also cause signal diffusion. Many detectable labels are bound to targets by non-covalent interactions such as protein-ligand binding or polynucleotide hybridization. Buffer changes after labeling may reduce the affinity between the target, binding agent, and detectable label, causing the various components to dissociate. Simple diffusion over a period of time, such as several days, may also cause dissociation between target, binding agent, and detectable label, rendering the signal diffuse.
Other problem associated with currently available detection procedures, in particular immunodetection, is time consumption. It normally takes 1 to 3 hours at minimum to process a sample from the step of labeling of targets to detection of the label.
Prior art describes only a very few techniques which allow to overcome the above mentioned problems, but yet only partially. One example of such techniques is a method of catalyzed reporter deposition (CARD) described in U.S. Pat. No. 5,863,748; 5,688,966; 5,767,287; 5,731,158; 5,583,001, 5,196,306, 6,372,937 or 6,593,100. This method utilizes so-called “analyte-dependent enzyme activation system” (ADEAS) to catalyze the deposition of a detectable label onto the solid phase of an assay platform. In the assay format, an enzyme comprised by the ADEAS reacts with a conjugate consisting of a detectably labeled substrate specific for the enzyme. When the enzyme and the conjugate react, an activated conjugate is formed which deposits covalently at a site where a specific receptor for the activated conjugate is immobilized. Thus, because of the conjugate comprises a label it plays a role of a reporter which indicates the presence of a target in the site. Enzymatically deposited labels may be detected directly or indirectly. The method results in signal amplification and improved detection limits.
The CARD method may be used in assay formats, where the target to be detected is a receptor immobilized on a solid support, e.g. a membrane. Such assays formats include sandwich immunoassays and membrane based nucleic acid hybridization assays. The CARD method is also applicable to detection of biological targets e.g. by immunohystochemistry (IHC), as described in U.S. Pat. No. 6,593,100. The method described in U.S. Pat. No. 6,593,100 utilizes a reaction of horse radish peroxidase (HRP) with a labeled conjugate comprising a HRP substrate in the presence of an enhancer. Both HRP substrate and enhancer are derivatives of phenol. Upon reaction with HRP the HRP substrate becomes activated and binds to receptor sites of the sample which are typically represented by low abundance aromatic amino acid residues of proteins.
Despite of having a relatively good sensitivity of detection of target molecules in samples compared to many other currently available methods, in particular methods for immunohistochemical detection of targets, the CARD method does not fully solve the other method's problems, for example strong background staining, still insufficient sensitivity and time consumption, which burden the method in cases of histological samples improperly proceeded prior the staining or those having a low level of target expression.
Recently, it has been described another HRP-based amplification method allowing detection of low abundance target molecules in IHC samples (WO2009036760). The method utilizes DAB not as a chromogenic substrate of HRP, but as a cross-linking agent of other HRP substrates. According to WO2009036760 DAB mediates HRP-mediated deposition of other HRP substrates, itself being not deposited DAB, i.e. there is no is characteristic visible brownish deposits is formed under conditions of the described deposition. The deposits of the other HRP substrate are detectable because they comprise a detectable label associated with the HRP substrate molecule, e.g. they may be detected in steps following the deposition step. The method provides for a strong amplification of a signal of the deposited HRP substrate, which makes the sensitivity of the method to be comparable with the CSA method, but compared to the latter method the new method advantageously provides no background labeling. Among other advantages of this new method, it is worth to mention that the speed of the detection procedure is much faster than either traditional DAB or biotinyl-tyramide detection procedure.